Kaitlin Olson, Rob McElhenney Welcome a Son

There'll be a new regular on the set of It's Always Sunny in Philadelphia this fall.

Costars Kaitlin Olson and Rob McElhenney welcomed their first child, son Axel Lee McElhenney, at 12:52 p.m. on Wednesday, Sept. 1 at their California home, they tell PEOPLE exclusively. He weighed in at 8 lbs., 7 oz.

"I went into labor at the Phillies/Dodgers game on Tuesday night," says Olson. "As soon as Ryan Howard hit a three-run home run, we felt comfortable leaving to go have our baby. We love him like crazy, even though he made us miss the second half of the game."

"Rob and I are so excited it's stupid," Olson, 35, said at the time. "The last couple years have been pretty amazing for us and just when we thought things couldn't get any better, the greatest little thing in the world happened! We're totally beside ourselves."

The FX cult comedy hit, which McElhenney created and executive produces, will return for its sixth season on Sept. 16.